As power storage devices, there are known secondary batteries, such as lithium ion batteries, nickel metal hydride batteries, and lead acid storage batteries, and electrochemical capacitors, such as electric double layer capacitors. Due to miniaturization or installation space restriction of mobile devices, for example, further miniaturization of power storage devices is sought, and therefore lithium ion batteries having high energy density are receiving attention. As packaging materials for lithium ion batteries, metal cans have been used. However, there is a trend recently toward use of multilayer films because of their light weight, high heat dissipation, and low production cost.
Lithium ion batteries using such a multilayer film as a packaging material have a structure of covering battery contents (a positive electrode, a separator, a negative electrode, an electrolyte, etc.) with a packaging material including an aluminum foil layer to thereby prevent entry of moisture. Lithium ion batteries having such a structure are referred to as aluminum laminated lithium ion batteries.
As an example of the aluminum laminated lithium ion batteries, embossed lithium ion batteries are known. In such an embossed lithium ion battery, a recess is formed in part of a packaging material by cold forming to store the battery contents in the recess, and the rest of the packaging material is folded back, with the edge portions being sealed by heat sealing (hereinafter, such a battery may be referred to as “single side formed battery”). In recent years, lithium ion batteries are also produced for the purpose of increasing energy density. In such a lithium ion battery, recesses are formed in two packaging materials to be bonded together to store more battery contents (hereinafter, such a battery may be referred to as “double side formed batteries”). Such a double side formed battery suffers from a problem of alignment difficulty in bonding the packaging materials to each other. However, for a single side formed battery to obtain energy density equivalent to that of a double side formed battery, a deeper recess is required to be formed.
The energy density of a lithium ion battery becomes higher as a deeper recess is formed by cold forming. However, forming a deeper recess is likely to create pinholes or cause breaking in the packaging material during forming, resulting in deterioration in formability. As a measure against this, a biaxially oriented polyamide film, such as a biaxially oriented nylon film (hereinafter, may be referred to as “biaxially oriented Ny film”), is used for a base material layer of a packaging material to protect the metal foil. However, such a biaxially oriented Ny film has low resistance against an electrolyte that is a contents of the lithium ion battery. When an electrolyte is contacted to the biaxially oriented Ny film during injection of the electrolyte in producing a lithium ion battery, the biaxially oriented Ny film is dissolved, resulting in a poor appearance.
To cope with this, there is proposed a packaging material having a base material layer whose surface is imparted with electrolytic resistance (e.g., refer to PTL 1). In this packaging material, a biaxially oriented polyethylene terephthalate film (hereinafter, may be referred to as “biaxially oriented PET film”) is further laminated outside the biaxially oriented Ny film.